Conservative leader Kemi Badenoch said: “A violent criminal doesn’t become less dangerous just because the government has run out of prison cells.”
She added: “[Labour’s] answer is to let dangerous offenders out even earlier instead of building the prison spaces we need.”
Labour Party chairwoman Bridget Phillipson said: “The Tories are taking the public for fools. Across 14 years in government, they added just 500 prison places and closed 23 prisons.
“Now Kemi Badenoch expects the British people to believe they’ll build 50,000.”
Meanwhile, the Prison Governors’ Association said “long-term investment” was needed and warned a “staffing crisis” would pose challenges to delivering new prisons.
If the party wins power, the Tories said they would introduce legislation early in the new Parliament to override planning and procurement barriers, allowing the process to be “fast-tracked”.
It said the use of private finance would help speed up delivery and control costs.
There are currently around 89,000 useable prison places in England and Wales, according to the latest government figures.
The Tories said their proposals would increase this figure by more than 50%, with the vast majority of new places expected within the party’s first Parliament in government – up to five years – and the full increase delivered within seven years.
The £2bn a year cost would be funded through savings in other areas, including cutting the aid budget by £1bn and reducing the budget of the Department for Culture Media and Sport by £1bn.
The Labour government has promised to add 14,000 extra prison space by 2031.
The Tories argued increasing prison places would cut crime and prevent reoffending by keeping serious criminals off the streets.
Shadow justice secretary Nick Timothy said: “Prison works. It works by punishing and deterring, but most importantly by incapacitating.
“Prison should keep violent and repeat offenders away from the law-abiding public.”